In a recent clinical trial led by Co-Leader Dr. Smith, Cabozantinib (CABO; XL-184) showed unprecedented bone scan responses in men with castration-resistant prostate cancer (CRPC). Although marked responses are seen, patients eventually progress and about 30% of patients do not respond. CABO is a multi-tyrosine kinase inhibitor with greatest activity against MET, VEGFR2 and RET, which have been implicated in prostate cancer (PCa) progression and the bone microenvironment. Using preclinical models we have found that some PCas show differential sensitivity to CABO when in bone versus soft tissue. Furthermore, through integrative sequencing, we have found that MET activation compensates for loss of androgen receptor (AR) signaling in CRPC. These clinical and pre-clinical results provide a compelling rationale for studying the role of both the tumor itself and the tumor microenvironment in predicting tumor sensitivity and resistance to CABO. Hence, the overarching goal of this proposal is to leverage an ongoing investigator-initiated clinical trial of CABO and use in vitro and in vivo modeling to reverse-engineer sensitivity and resistance mechanisms using a bedside to bench approach. Thus, we propose the following Specific Aims: Aim 1. Determine if clinical response to CABO in men with CRPC is associated with inhibition of META/EGFR/RET signaling. Clinical response to CABO with AR, MET, VEGF and RET signaling activity in bone and soft tissue will be evaluated. A novel method of diffusion-weighted MRI to evaluate bone response will be used and flash frozen pre-treatment and on-study biopsies for interrogation in Aim 3 will be obtained. Aim 2. Assess the role of the microenvironment in conferring sensitivity and resistance to CABO. The response to CABO In multiple prostate cancer cell lines with varied AR and MET pathway activity In the context of tissue culture, soft tissue sites and bone sites will be evaluated. Activation and targeting of CABO-sensitive pathways (MET, VEGFR, RET and downstream effectors and AR signaling) will be evaluated. Tumors will be subjected to RNA-seq analysis to identify novel predictors of sensitivity and resistance. Aim 3: Interrogate the role of tumor genotype and phenotype in conferring sensitivity and resistance to CABO. An integrative sequencing strategy on the pre- and on- treatment biopsies to characterize genomic and transcriptomic predictors of sensitivity and resistance to CABO will be performed. CABO-resistant cell models will be developed and subjected to integrative sequencing to identify mechanisms of acquired resistance. Candidate mediators of resistance will then be tested in appropriate in vitro and in vivo models.